Large magneto-optical Kerr effect in noncollinear antiferromagnets<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Mn</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mi>X</mml:mi></mml:mrow><mml:mspace width="0.28em"/><mml:mo>(</mml:mo><mml:mi>X</mml:mi><mml:mo>=</mml:mo><mml:mi>Rh</mml:mi><mml:mo>,</mml:mo><mml:mspace width="0.28em"/><mml:mi>Ir</mml:mi><mml:mo>,</mml:mo><mml:mspace width="0.28em"/><mml:mi>Pt</mml:mi><mml:mo>)</mml:mo></mml:math>

Magneto-optical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using first-principles density functional theory, we demonstrate large magneto-optical Kerr effect in high-temperature noncollinear antiferromagnets ${\mathrm{Mn}}_{3}X\phantom{\rule{0.28em}{0ex}}(X=\mathrm{Rh},\phantom{\rule{0.28em}{0ex}}\mathrm{Ir},\phantom{\rule{0.28em}{0ex}}\mathrm{Pt})$, in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition-metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of band double degeneracy due to the absence of spatial symmetry in the ${\mathrm{Mn}}_{3}X$ noncollinear antiferromagnets which together with the time-reversal symmetry would preserve the Kramers theorem. Our results indicate that ${\mathrm{Mn}}_{3}X$ would provide a rare material platform for exploration of subtle magneto-optical phenomena in noncollinear magnetic materials without net magnetization.